Commercial lighting consists primarily of gas-filled fluorescent lamps, which are a family of artificial light sources that generate light by sending an electrical discharge through an ionized gas (i.e., a plasma). Typically, these lamps are filled with noble gases such as argon, neon, krypton, and xenon, or a mixture of such gases. Many lamps are also filled with additional materials such as mercury, sodium, or metal halides. In operation, the gas is ionized, electrons are accelerated by an electrical field within the lamp, and the electrons collide with gas and metal atoms. Ultraviolet radiation resulting from these collisions is converted to visible light by a fluorescent coating on the inside of the lamp's glass surface.
There are three primary categories of gas-discharge fluorescent lamps. Low-pressure discharge lamps are commonly used as, for example, office lighting. Each low-pressure discharge lamp has a luminous efficacy of approximately 100 lumens per watt (“lm/W”) or more. For example, low-pressure sodium-vapor lamps are the most efficient of the gas-discharge type fluorescent lamps and have a luminous efficacy of up to 200 lm/W. However, the efficiency of the low-pressure sodium-vapor lamps is offset by poor color rendering. In many instances, nearly monochromatic yellow light is produced.
High-pressure discharge fluorescent lamps, such as metal halide lamps, are capable of producing an output which is approximately white light and have a luminous efficacy of approximately 100 lm/W. Applications of high-pressure discharge fluorescent lamps include indoor lighting of commercial buildings, parking lot lighting, and venue lighting (e.g., sporting venue lighting). High-pressure sodium-vapor lamps have a luminous efficacy of up to 150 lm/W and produce a broader light spectrum (i.e., produce more wavelengths of visible light) than the low-pressure sodium-vapor lamps.
High-intensity discharge (“HID”) fluorescent lamps are a type of lamp which produces light by way of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tube. The tube is filled with gas and metal salts. The gas facilitates the arc's initial strike, and once the arc is started, it heats and evaporates the metal salts to form a plasma that increases the intensity of the light produced.